To ensure your migration runs as smoothly as possible, you should first decide which type of migration most suits your needs. You can perform either a one-time, “big bang” migration (cutover) OR several staged groups (co-existence). You may choose a cutover approach if you have a small number of users and you want to move everyone all at once to reduce the amount of administrative overhead. This is also a good option if your users are experienced and won’t require a lot of training on the new target domain client. Inform users that they may not have all their old email right away.
The co-existence approach is a good option if you need to train your users on the new mail system. This minimizes the number of users calling the helpdesk. It also allows you to schedule training sessions and migrations together, managing expectations for both your IT staff and your end-users.
Before migrating or creating mailboxes, you need to implement a mail routing strategy that will enable users to send and receive mail from both internal and external sources. There are different strategies to do this depending on how you want mail directed to and from the internet.
During the co-existence period of on-premises mail and hosted mail, you need to decide which system will initially receive the email traffic for your organization. To make the best decision for your organization, please review the Microsoft documentation associated with your target email service: